A system using an optical type digital audio disc (called a compact disc) is a disc system which can reproduce stereophonic music of high quality. If digital data such as display data, program data or the like other than stereophonic music can be reproduced by this disc system, by equipping a display device, it is possible to realize an apparatus for reproducing visual information such as graphic diagrams and statistics, drawings by still pictures, and the like, and to realize a video game apparatus. In addition, by recording sound information and image information, it is possible to realize a complete art collection, a travellers' guide, education system, and the like by use of both information, so that the range of applications of compact disc systems can be extended. In this case, an amount of audio data which can be transmitted decreases since still picture data is recorded.
One of such reduction is a decrease in number of bits, namely, one sample consists of eight bits as compared with the existing compact disc in which one sample consists of sixteen bits. In other words, a dynamic range is reduced. For example, the dynamic range of 96 dB is obtained in case of 16 bits, while only the dynamic range of 48 dB is derived in case of eight bits. However, for this problem, a value of 60 dB or 78 dB or the like which is near the inherent dynamic range can be obtained using a D-PCM method or an analog compressing/expanding method.
Another problem relates to a decrease in transmission band due to reduced sampling data which can be transmitted. In the existing compact disc, the transmission band of 20 kHz is derived. However, the transmission band is reduced to about 10 kHz when the sampling data is reduced because of the recording of the still picture data. Even in this case, a large inconvenience for the reproduction of a voice will not be caused. However, the transmission band of about 15 kHz is desirable to reproduce a music.
It is an object of the present invention to provide a data transmission system which can reproduce digital data other than stereophonic music signals by use of the existing disc reproducing apparatus as it is.